This invention relates to a control circuit for a thermal printing head arranged in a facsimile system or the like and more particularly to a printing pulse control circuit which controls the delivery of printing pulses so that heat generation in heat generator elements constituting a thermal printing head is so controlled as to effect uniform printing on thermal printing paper.
A variety of types of printing are available in facsimile but some of them do not satisfy compatibility with scanning systems, economical advantages and maintenance requirements. Accordingly, of many printing types, electrostatic printing, thermal printing, ink jet printing, current conduction type thermal printing, discharge puncture printing or electrolysis printing is typically used for facsimile. Today, used for high speed printers (adapted for GIII standard) calling for high speed operation and high resolution is electrostatic printing because of its high printing speed and ability to produce clear printing pictures although an operational arrangement for practicing the electrostatic printing is rather complicated and has somewhat difficulties with maintenance; and used for medium speed printers (adapted for GII standard) is thermal printing which satisfies economical advantages and easy maintenance and dispenses with development and fixing devices. Currently, improvements in printing paper and printing head make way for employment for high speed printers of the thermal printing whose economical advantages and easy maintenance are highly evaluated.
A printing head adapted for thermal printing has a number of thermal resistor elements arranged in line, as disclosed in U.S. Pat. No. 3,984,844, for example. The head is connected with a drive circuit adapted to sequentially pass signal currents in response to printing signals through the thermal resistor elements, so that printing is effected on thermal printing paper which is in contact with the head. The multiple thermal resistor elements adapted for the thermal printing head is fabricated in the form of either a thick film or a thin film, and the drive circuit may be rendered compatible with either type of head.
As described above, in a facsimile system adopting thermal printing, a number of thermal resistor elements are aligned to constitute a thermal printing head, and desired thermal resistor elements are heated and brought into contact with thermal printing paper to color the paper, thereby effecting printing. In a thermal printing based on the above type of printing, the thermal printing head, on the one hand, is required to be of high thermal response, and high heat-proof and wearproof properties and the thermal printing paper, on the other hand, is required to be of excellent coloring characteristics and high printing consistency. In the past, in order to improve the above various characteristics, attempts have been made to improve the response of the thermal resistor elements constituting the thermal printing head and the resistor elements thin.
However, conventional technologies, though having experienced the aforementioned improvements, disdadvantageously faced uneven printing consistency due to temperature variation in the thermal printing head, due to the difference in printing data resulting from black record information or white record information for the preceding line, and due to the time difference between completion of printing operation for the preceding line and initiation of printing operation for the present line.
FIG. 1 illustrates a conventional control circuit adapted to generate pulses for controlling heat generation in thermal resistor elements of a thermal printing head 7. The control circuit comprises NAND circuits 1 and 2 connected to input terminals S and C, AND circuits 3 and 5 respectively connected to an input terminal D.sub.1 and an output terminal D.sub.o, open-collector type inverter drive circuits 6a to 6n, and latch circuits 4a to 4n connected to the output of the NAND circuit 2 and the output of the AND circuit 3. The terminals S, C, D.sub.1 and D.sub.o serve as a strobe signal input terminal, a clock signal input terminal, a data input terminal and a latch output terminal, respectively. Denoted by Q.sub.1 to Q.sub.n are output signal terminals which have connections to heat generation control terminals of the thermal resistor elements of the thermal printing head.
With the control circuit, input information applied to the data input terminal D.sub.1 is serially stored in the latch circuits 4a to 4n in synchronism with a clock signal applied to the terminal C, and when a strobe signal (a pulse wave) is applied to the terminal S, the output or outputs of one or ones of the inverter drive circuits 6a to 6n corresponding to the serial information of "1" are rendered "0". Thus, the thermal resistor elements of the thermal printing head connected with the output terminals Q.sub.1 to Q.sub.n, one or ones corresponding to the "0" output bit or bits of the inverter drive circuits are applied with currents and portions of printing paper in contact with the thermal resistor elements in current conduction color to effect printing of given information.
However, since with the conventional control circuit shown in FIG. 1, the pulse width of current applied to the thermal resistor element is controlled only by the strobe signal from the terminal S, unevenness of printing consistency due to temperature difference in the thermal printing head occurs upon on-off switching of the strobe signal and in addition, when "black" printing information follows "white" printing information, printing consistency of the "black" printing information is disadvantageously made pale.